Transport networks serve for the physical layer transport of high bitrate tributary signals. In particular, the signals transmitted over a transport network are encoded and multiplexed into a continuous bit stream structured into frames of the same length. Within this constant bitrate bit stream, the frames are repeated periodically with a frame repetition rate of typically 8 kHz and are structured according to a multiplexing hierarchy. An example of such a multiplexing hierarchy is SDH (Synchronous Digital Hierarchy, see ITU-T G.707 10/2000) where the frames are termed synchronous transport modules of size N (STM-N, where N=1, 4, 16, 64, or 256). The frames have a section overhead and contain at least one higher order multiplexing unit called virtual container VC-4. A VC-4 can either directly carry a tributary signal or a number of lower order multiplexing units like VC-12 or VC-3, which then carry tributary signals.
Virtual containers are transmitted from source to sink through an SDH network and therefore represent a “logical” path through the network. The sequence of identical VCs having the same position in subsequent frames forms a traffic stream along that path. Each VC contains a path overhead (POH) and a payload section referred to as container (C). The US equivalent of SDH is known as SONET (Synchronous Optical Network). Another well known transport network with similar multiplexing units is the recently defined Optical Transport Network OTN; see ITU-T G.709, 02/2001.
The transport network itself consists of a number of physically interconnected network elements such as crossconnects and add/drop multiplexers. Traditional transport networks are managed centrally. This means that a central network manager has the overview about the topology and status of the network and if a customer desires a new connection for a tributary signal, the network operator manually establishes via his network management system a corresponding path through the transport network. Thus, paths through a centrally managed network are created under the control of the central network management system, which instructs all affected network elements (potentially using intermediate lower level network management facilities) to switch corresponding crossconnections to establish the new path.
In label switched packet networks, as opposed to transport networks, paths, which are referred to as label switched paths (LSPs) in this context, are created automatically using MPLS (Multi-Protocol Label Switching) signaling. Network devices in such packet switched networks use routing protocols such as OSPF and BGP to update and synchronize their local routing information. The fundamental difference between transport networks and packet networks where MPLS applies is, that in packet networks statistical multiplexing is used allowing over-subscription of links and that an LSP can be established without using any bandwidth. However, in transport networks, if a path is established, then by definition the full bandwidth requested by the path is consumed, independent of whether traffic is transmitted over this path or not. An LSP can be established in MPLS but not used, whereas this is not possible in transport networks. Due to the dynamic and inconstant data rate in a packet network, the number and bandwidth of LSPs does not necessarily correlate with the idle capacity of the physical link over which the LSPs lead.
However, the obvious advantages of label switched packet networks in terms of flexibility and failure resistance have lead to the development of automatically switched optical transport networks (ASONs). This development has culminated in the definition of a new signaling protocol for optical networks which is known as GMPLS (Generalized Multi-Protocol Label Switching). The underlying principle is that each network element has its own GMPLS controller. The GMPLS controllers in the network communicate over a dedicated Ethernet data network with each other to coordinate a path set-up and configure their corresponding network elements accordingly to automatically establish a dynamically agreed paths. Each GMPLS controller must therefore have a complete knowledge of the topology and status of the entire transport network. An OSPF protocol (Open Shortest Path First), extended to the particular needs of a GMPLS-controlled transport network, is used to communicate (or “advertise”) the status of the transport network from one GMPLS controller to the other. Each controller has a database where it stores the topology data of the network according to its latest knowledge.
Since in a GMPLS-controlled transport network, the network management or control plane, as it is referred to in this context, is distributed among the entire network, the network operator has no central tool anymore that might give him an overview over his network. It would be possible to link a presentation tool to the GMPLS controller of an arbitrary network element and display the local topology and status information stored by this particular network element to the operator. However, the local database contents, as it relates to the device of a particular equipment vendor, is interspersed with vendor-specific extras. Moreover, the operator would have to rely on the proper functioning of this particular implementation of the database and it would not be possible to check the database contents against the databases of other GMPLS controllers in the network.
A need exists therefore for an independent device for determining and displaying the topology and status of an automatically switched optical transport network.
From U.S. Pat. No. 5,926,463, an apparatus for viewing and managing a configuration of a computer network is known. The apparatus polls a plurality of switches and routers present in a packet switched network to obtain copies of information stored in databases on the switches and routers. It determines from this combined database the status of the network and displays physical connectivity and status of the network graphically to the user. This tool is, however, suited for packet networks only, and requires a vendor-independent definition of the database structure stored in each network device. Moreover, a protocol for the polling mechanism is required, which is, however, not foreseen in GMPLS networks.
From U.S. Pat. No. 5,917,808 a test instrument for testing local area networks (LANs) is known, which uses passive monitoring and allows to identify network device types on a LAN operating according to the TCP/IP protocol suite. The test instrument, when coupled to the LAN, passively receives traffic in the form of frames that are being sent between the nodes on the LAN. The frames being sent may contain information that may be uniquely associated with specific types of network devices such as servers, routers, or printers. A frame processor collects and extracts the frame information from the frames, including the message type and source IP address. The frame information is compared against sets of frame types, with each set of frame types uniquely associated with one of the network device types. The device types as detected are added to a station database and displayed graphically to a user. This instrument, however, does not seem to be of any use in an automatically switched optical transport network of the type described above.
It is therefore an object of the present invention to provide a device and corresponding method for determining and displaying the topology and status of an automatically switched optical transport network.